The invention relates to a casting machine which has at least one exactly and reproducibly equipped, water-cooled mold for continuously casting a vertical ingot in the magnetic field of a closed peripheral, partially shielded inductor, cooling water channels directed at the ingot at an acute angle via at least one guiding, deflecting surface on which a film of water forms, and for each mold a corresponding dummy base that can be lowered. Further, the invention relates to a process for cooling an ingot in a casting machine.
Continuous chill casting of metals produces slabs or logs that are several meters long and serve as starting material for various subsequent processing steps such as extrusion, rolling or forging.
The most important past of a continuous casting machine are the molds which in conventional processes determine the cross-section of the cast ingot. A casting machine is, depending on the number of slabs or logs being cast, fitted with the corresponding number of dummy bases that can be lowered and are attached to a casting platform.
As the mold slowly starts to fill with molten metal, the metal on the dummy bases starts to solidify. The latter are cooled and lowered at such a rate that the solidus line of the solidifying metal always remains within the frame of the mold. The ingots, the solidification of which is accelerated by water-cooling, increase in length downwards at the same rate as the dummy bases are lowered i.e drop rate. For a given length of ingot the casting process is carried out without interruption.
Among the most important parameters in continuous casting are a correctly controlled drop rate and the cooling of the metal at the right place with the right intensity. These parameters have a strong influence on the surface of the cast ingot. Unfavorable control of these parameters can lead to segregation, liquid metal penetrating the solidified skin, the latter tearing open, or chalk deposition. Electromagnetic casting (EMC), which has reached industrial maturity only in recent times, is based on complete elimination of mechanical contact between the mold and the solidifying metal. The liquid metal is kept exactly in the cross-sectional shape of the ingot by means of controllable electromagnetic forces.
The EMC process not only enables a homogeneous internal structure to be achieved but also a smooth surface of solidified metal, which leads to better physical and chemical properties in extrusion billets, forging blanks and rolling slabs. Expensive processing steps such as the removal of the surface skin or edge trimming are no longer necessary with the EMC process.
Very important in electromagnetic casting is the start-up phase as the solidification front is kept within a narrow height range of about 10 mm. This is necessary because the electromagnetic forces have to compensate for the metallostatic force of the melt above the solidification front. For that reason it is essential to have complete control of the cooling, especially during start-up. The drop rate and the cooling for specific alloys and ingot dimensions have to be optimized on a time basis.
Curvature of the ingot bottom and local crack formation can to a large degree be eliminated if the effect of thermal shock and the intensity of the cooling water can be reduced:
By using cooling water containing carbon dioxide the intensity of cooling can be reduced by about a factor of 5. The use of water containing CO.sub.2 is however accompanied by some disadvantages. The carbon dioxide has to be contained in gas bottles under pressure, transported and stored. Further, the cooling water containing CO.sub.2 has to be kept under high pressure until shortly before use, which in turn leads to higher expenditures in terms of design and materials.
Another varient makes use of pulsed, sprayed cooling water at least during the start-up phase. This method has proved itself with most aluminum alloys; in the case of hard alloys, however, hair-like cracks can form.
The downwards wedge-shaped electromagnetic shield for known EMC machine molds fulfils two functions simultaneously:
The material used for the shield, stainless steel, absorbs the electromagnetic forces forming the ingot increasingly with increasing thickness. This leads to additional heating.
The polished outer surface of an inclined pan of the shield acts first as a surface to guide the cooling water, and such that initially a film of cooling water forms on that surface, then a curtain of water is sprayed onto the ingot. As a side effect, the shield is cooled by the impinging water. Stainless steel for example is a particularly poor thermal conductor.
Consequently there are some problems with conventional EMC molds:
A chalk deposit forms on the polished outer face of the electromagnetic shield, the surface guiding the cooling water, which leads to an imperfect film of cooling water and inadequate cooling of the EMC shield. As this cooling must be adequate, large maintenance costs are unavoidable.
The EMC shield is rigidly attached to the mold, therefore the position of the surface guiding the cooling water can not be altered.
The various components of the mold are made of aluminum, iron, and copper, which can lead to corrosion problems.